146 T. 0. Mendenhall — Use of Planes and Knife-edges 



shown below include this effect, of course, it being eliminated 

 only from their mean. 



The two sets showu were made for the purpose of determin- 

 ing a " pressure coefficient," the first being made in air at a 

 pressure of 25 mm and the second at a pressure of 600 mm . It 

 will be seen that in no case does the variation from the mean 

 amount to as much as one part in a million, a constancy which 

 leaves little to be desired. 



Pres. 25 mm . 





Pres. 600 m! 



Period v. 





Period v. 



•5006904 — 4 



•500 



7368 + 4 



901 — 1 



• 



74 — 2 



898 + 2 





71 + I 



900 





73 — 1 



902 — 2 





72 



899 + 1 





75 — 3 



901 — 1 





73 — 1 



898 + 2 





71 + 1 



896 + 4 





70 + 2 



902 — 2 





68 + 4 



899 + 1 





69 + 3 



900 





75 — 3 



ean -5006900 



Mean -500 



7372 



The advantages of the new form of pendulum will be made 

 evident on an examination of its application to the differential 

 method or use of a so-called invariable pendulum as well as to 

 the reversible form for absolute determinations of gravity. 



There is an advantage in the matter of construction. It is 

 not easy to insert the knife-edge in the head of the pendulum 

 so that it shall be at light angles to the axis of symmetry of 

 mass. The plane used in its stead may be accurately adjusted 

 by simple optical methods. 



The advantage of the plane in the matter of permanency or 

 invariability is so evident as hardly to need remark. The 

 knife-edge is usually the most delicate part of a pendulum, 

 that most liable to be injured and generally incapable of being 

 repaired when once damaged. In an invariable pendulum the 

 knife-edge cannot be reground or replaced by another, with- 

 out destroying the identity of the whole so that swings at 

 different places or times are no longer comparable with each 

 other. A pendulum carrying a plane instead of a knife-edge 

 is evidently vastly less liable to accidental injury and is enti- 

 tled in a much greater degree to the name "invariable." 

 The knife-edge being no longer an integral part of the vibrat- 

 ing mass can be reground or replaced at will. In fact in prac- 

 tice it is desirable to have several knife-edges and in an exten- 

 sive pendulum campaign a " standard edge " will be used 



